Place the onions, butter, oil and thyme in a large saucepan over medium heat, cover and cook for 35 minutes, stirring occasionally, until soft and golden. Add the flour and cook, stirring, for 3 minutes. Add the brandy and cook, stirring, for 1 minute. Add the mustard, stock and water and allow to simmer for 15 minutes.
Ladle the soup into bowls and serve with toasted cheese sandwiches or parmesan toasts. Serves 4–6.
*The secret to making this soup is to brown the onions very well, until they are a deep golden colour, a lovely soft texture, and really sweet and flavoursome. This is quite a rich soup and is ideal served as a meal with toasted bread or sandwiches on the side. If serving as a starter, offer just a small bowl of French onion soup or make the main course a light one.